1. Field of the Invention
The present invention relates to a toner, and more particularly, to a toner that may be fixed at low temperature by adding a crystalline polyester resin to a main binder resin.
2. Description of the Related Art
An inkjet printer and a laser printer have been currently widely used. The inkjet printer is typically used as a personal printer due to a slow print speed. On the other hand, the laser printer is suitable to be used in an office environment in which it is linked to several network computers to print a large volume. In such a laser printer, a latent image is created on a photoconductor (OPC) using a laser. Thereafter, a toner is attracted to the latent image of the photoconductor due to the difference of the electric potential, and then the latent image is transferred onto a receiving material, such as a sheet of paper, resulting in a visible image.
A laser printer that has been currently developed or commercially available is a black and white dry printer. In the black and white dry printer, a toner is charged by friction and then transferred to a latent image of a photoconductor due to the difference of the electric potential. Even though the black and white dry printer causes an environmental problem such as dust generation due to toner particles, the black and white dry printer has an advantage in that it may be easily manufactured in a small size at a low cost.
Generally, a dry toner includes a pigment, a charge control agent (CCA) that adjusts the amount of the charges of the toner, a binder resin that binds the pigment and the charge control agent, and a releasing agent that facilitates the separation of-a transfer medium after the transfer. Furthermore, a surface additive is added to the surfaces of toner particles to impart functionality to the toner particles or to enhance the physical properties of the toner particles.
A toner image formed on the surface of a transfer medium is finally fixed by a contact or non-contact developing method. With respect to the former, a heat roller fixing method is mainly used, in which a toner image formed on the surface of a transfer medium is fixed by a heat roller set to 120 to 180° C.
About 80 to 90% of a dry toner comprises a binder resin as described above. In such a dry toner, an offset resistance and a fixing property on a transfer medium are required. The term, “offset,” as used herein, indicates a phenomenon that when a toner image transferred from the surface of a drum to the surface of a transfer medium is thermally fixed due to attachment of the toner image on a heat roller, the toner image is printed on an unwanted surface of a transfer medium during subsequent printing. The offset printing may be divided into a cold offset that occurs at a low temperature range and a hot offset that occurs at a high temperature range. The above-described properties of a toner are mainly determined by the properties of a binder resin used in preparation of the toner. Generally, if the melting temperature of a binder resin is low, a fixing property is effective, but an offset resistance is ineffective. On the other hand, if the melting temperature of a binder resin is high, an offset resistance is effective but a fixing property is ineffective.
Since a binder resin with a low melting temperature also has a low glass transition temperature (Tg) (also called a softening point), repeated developing by a heat roller fixing method may cause deterioration/aggravation and ineffective storage stability of a toner due to a mechanical friction force. To prevent these problems, use of a binder resin with a high glass transition temperature may be considered. However, in this case, due to the low thermal efficiency of the binder resin, a toner image is incompletely fixed and a high fixing temperature is required. For this reason, an intense heat is used for fixing, which may cause aging of a fixing apparatus. Furthermore, an intense heat may cause paper curling, excess energy consumption, and insufficient pulverizability, thus decreasing production efficiency. For example, with respect to a styrene based copolymer resin generally used as a binder resin for a toner, a fixing temperature varies, depending on the composition of the binder resin, the outer diameter of a heat roller, and a print speed. However, in a general case, since a fixing temperature may be as high as about 160° C., an intense heat is used during fixing, which may cause the above-described problems.
Meanwhile, in an attempt to enhance an offset resistance, a method was implemented in which the surface of a heat roller was coated with fluorinated resin or silicon oil. However, this method increased the size, structural complexity, and manufacturing cost of a fixing apparatus. Even though an offset resistance was enhanced, the fixing temperature of a binder resin was still high.
In this regard, attempts have been made to decrease the fixing temperature of a toner simultaneously with satisfying a fixing property and an offset resistance.
Japanese Patent Laid-Open Publication No. Sho. 60-134,248 discloses a partially crosslinked binder resin, one particle of which has a crosslinked structure and a low molecular weight linear structure obtained through a two step polymerization. However, the offset resistance and the fixing property of a toner tend to be inversely related according to the crosslinking density of the binder resin. That is, a low crosslinking density ensures an effective fixing property. However, it is difficult to maintain an appropriate gel content, which may lower an offset resistance. On the other hand, a high crosslinking density ensures an effective offset resistance, but lowers a fixing property. Also, there arises a problem in that such a crosslinking density is not easily adjusted.
Meanwhile, as an attempt to enhance both the offset resistance and the low temperature fixing property, a study has been done on development of a capsule toner having an inner core and an outer shell that are different in terms of a material and a structure. In this regard, according to a known technique that ensures low temperature fixing simultaneously with satisfying a long-term storage stability, a liquid or a low melting point wax that may be easily plasticized is used as the inner core of the capsule toner and a strong material is used as the outer shell of the capsule toner. However, such a capsule toner has an ineffective fixing property. To overcome this problem, use of a liquid core can be considered. In this case, however, if the strength of the shell is weak, a toner is broken down in a developing apparatus, thus causing internal contamination. In this regard, a high melting point polymer may be used as a shell material. However, the high melting point polymer cannot be used for low temperature fixing. In addition, the capsule toner involves a complicated preparation process and requires a separate additional apparatus, thus incurring an additional cost.
There is another method of using common polyester as a binder resin. However, polyester commonly used as a toner binder has a glass transition temperature of about 61° C. and a melting temperature of about 160° C., which are insufficient to lower a fixing temperature. Therefore, using common polyester is not suitable for low temperature fixing.